1. Field of the Invention
The present invention relates to a spark plug having a noble-metal tip that is welded to each of a front end portion of a center electrode and a distal end portion of a ground electrode so as to form a spark discharge gap between the noble-metal tips.
2. Description of the Related Art
A spark plug is attached to an engine of an automobile or the like and used as an igniter for igniting an air-fuel mixture so as to drive the automobile or the like. Such a spark plug is generally configured as follows: an insulator holding a center electrode is fixedly fitted into a metallic shell having an externally threaded portion used to mount the spark plug to an engine; and a ground electrode is joined to a front end portion of the metallic shell such that a spark discharge gap is formed between opposed end portions of the center and ground electrodes.
The combustion performance of an engine greatly depends on the ignition performance of a spark plug. In order to maintain its ignition performance over a long period of time, a certain spark plug is configured such that a noble-metal tip formed of an iridium (Ir) alloy, a platinum (Pt) alloy, or the like, which exhibits excellent resistance to spark-induced consumption and oxidation-induced consumption, is welded to each of a front end portion of a center electrode and a distal end portion of a ground electrode.
For example, Patent Document 1 discloses a spark plug in which a tip formed of an iridium alloy is welded to each of a front end portion of a center electrode and a distal end portion of a ground electrode, whereby the spark plug exhibits excellent resistance to consumption.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2002-184551 (FIGS. 20, 25, and 26)
3. Problems to be Solved by the Invention
Although a spark plug may have noble-metal tips which exhibit excellent resistance to consumption, this does not necessarily mean that the spark plug itself exhibits excellent resistance to consumption and extended service life. Conceivably, such a spark plug fails to exhibit extended service life for the following reason. Spark discharges do not always occur at expected positions, and as a result, the noble-metal tip may become detached from a center electrode or a ground electrode (hereinafter also called a base metal). The present inventors have determined that spark discharges which concentrate on a weld metal zone where the noble-metal tip and the base metal are joined together can result in detachment of a noble metal tip, thereby causing unusual consumption of the weld metal zone.